Cements & Concrete Flashcards
What is the difference between cement and concrete?
Cement is an ingredient of concrete. Concrete is a mixture of aggregates (sand, gravel, or crushed stone) and paste (water and Portland cement).
What are the raw materials used in Portland cement production?
- Lime (CaO): Limestone, shale.
- Silica (SiO2): Clay, sand, shale.
- Alumina (Al2O3): Clay, fly ash, shale.
- Iron (Fe2O3): Clay, iron ore.
What are the types of Portland cement and their uses?
Type I: General use.
Type II: Used in water or soil with moderate sulfate levels or when heat build-up is a concern.
Type III: High early strength.
Type IV: Low heat.
Type V: Sulfate-resistant.
Type IA,IIA,IIIA: Air-entrained concrete.
What are the four stages of Portland cement production?
- Crushing and grinding raw materials.
- Blending materials in correct proportions.
- Burning the mix in a kiln.
- Grinding the clinker with gypsum.
What is the role of water in concrete?
- Chemical reaction with cement (hydration).
- Workability.
- Only 1/3 of the water is needed for the chemical reaction; the rest remains in pores, affecting porosity.
- Good for preventing plastic shrinkage
- Bad for permeability, strength, durability
What is hydration in concrete?
Hydration is the chemical reaction between cement compounds (C3S and C2S) and water, forming calcium silicate hydrate (CSH) and calcium hydroxide (CH)
What are the types of aggregates used in concrete?
- Coarse aggregates (gravel or crushed stone).
- Fine aggregates (sand).
What are admixtures, and what are their types?
Admixtures are chemicals or minerals added to concrete to modify its properties. Types include:
Set retarders.
Set accelerators.
Water reducers.
Air-entraining agents.
Fly ash, silica fume, and slag.
What is the slump test, and what does it measure?
The slump test measures the workability of fresh concrete. It involves filling a cone with concrete, removing the cone, and measuring the slump (settlement) of the concrete.
What factors affect the slump of concrete?
Water-cement ratio.
Paste content.
Aggregate size and shape.
What is air entrainment in concrete?
Air entrainment involves adding air-entraining agents to stabilize tiny bubbles in concrete, improving resistance to freezing and thawing cycles.
What is bleeding in concrete, and how can it be controlled?
Bleeding is the accumulation of water on the surface of freshly placed concrete. It can be controlled by:
Adjusting the mix design.
Reducing water content.
Adding more fines or air-entraining agents.
What causes plastic shrinkage cracking, and how can it be prevented?
Plastic shrinkage cracking occurs when water evaporates faster than it can reach the surface. Prevention methods include:
Controlling wind velocity.
Reducing concrete temperature.
Increasing surface humidity.
Using curing compounds or fiber reinforcement.
What is curing, and why is it important?
Curing is the process of maintaining adequate moisture and temperature for the chemical reaction of cement with water.
It is important because it ensures the concrete reaches its desired strength and durability.
What are the effects of temperature on curing?
- Higher temperatures accelerate curing but can reduce durability.
- Freezing within the first 24 hours can permanently damage concrete.
- The ideal curing temperature is around room temperature.
What is the role of fly ash in concrete?
Fly ash is a pozzolanic material that can replace part of the cement, improving workability, reducing heat of hydration, and enhancing durability.
What is silica fume, and how is it used in concrete?
Silica fume is a by-product of silicon production. It is used to replace part of the cement, improving strength and durability due to its fine particles.
What is slag, and how is it used in concrete?
Slag is a by-product of iron production, rapidly cooled in water. It is used as a cement replacement, improving workability and durability.
What are the properties of fresh concrete?
Workability: Ease of placement and resistance to segregation.
Consistency: Ability to flow.
What is the effect of aggregate size on concrete?
- Larger aggregates have less surface area, leading to a thicker paste coating and higher slump.
- Smaller aggregates have more surface area, leading to interlocking and lower slump.
What is the effect of angularity and surface texture of aggregates on concrete?
Angular and rough aggregates improve bonding but reduce workability. Smooth aggregates (like river gravel) improve workability but reduce bonding.
What are the remedies for bleeding in concrete?
- Adjust the mix design.
- Reduce water content.
- Add more fines or air-entraining agents.
- Improve grading of aggregates.
What are the three processes of cement manufacture?
- Wet process: Raw materials are ground wet and fed as a slurry.
- Dry process: Raw materials are ground dry and fed as a dry powder.
- Semi-dry process: Raw materials are ground dry and moistened to form nodules.
What is the significance of the slump test in concrete?
The slump test measures the consistency and workability of fresh concrete, ensuring it meets the desired specifications for placement and finishing.
What is the role of admixtures in concrete?
Admixtures are used to modify the properties of concrete, such as setting time, workability, strength, and durability.
